"Silacrowns" or "silacrown ethers" are macrocyclic multidentate ethers which resemble in structure and complexation properties a class of compounds known as "crown ethers", but differ in the replacement of a --C.sub.2 H.sub.4 -- group by a silicon group. U.S. Pat. No. 4,362,884 discloses the preparation and uses of such compounds. The present invention is directed to the preparation of modified silacrowns which have the ability to assist in the complexation of various cations.
Since 1967 when C. Pedersen discovered the crown ethers, literally thousands of applications have developed in which their ability to complex metal ions, solvate inorganic and organic salts in polar and non-polar solvents, and facilitate anionic reactions have been exploited. Much of this work has been reviewed in Synthetic Multidentate Macrocyclic Compounds by R. Izatt and J. Christiansen, Academic Press 1978. Two obstacles have prevented their wider utilization, particularly in commercial processes: current synthetic methods are extremely costly, and the materials have generally high levels of toxicity. These factors, coupled with the difficulty in separating the crown ethers during preparation by processes other than distillation, have hindered wider applications. An example is the acylation step in penicillin synthesis.
Although cyclic polyethyleneoxysilanes have been previously reported, the ring structures have fewer members than the silacrowns. The inside diameters of the ring structures are clearly smaller than lithium ions. R. Krieble, C. Burkhard, J. Am. Chem. Soc. 69, 2689 (1947). Because the ring structures are so small, these compounds cannot form complexes with cations.
As disclosed in U.S. Pat. No. 4,362,884, silacrowns exhibit complexation properties remarkably similar to crown ethers. A specific example is dimethylsila-14-crown-5. ##STR2## The name indicates the substituents on silicon, the number of members in the ring, and the number of oxygens. This compound may be compared to the corresponding crown ether, 15-crown-5. Although there is one less member in the ring for the silacrown, the longer silicon-oxygen bonds result in an O-Si-O unit that has 75 percent of the bond length of an O-CH.sub.2 -CH.sub.2 -O unit. Simple addition of bond lengths indicates an overall reduction in macrocycle circumference of 4.5 percent when compared to 15-crown-5.
Hereinafter, the absence of a prefix in a silacrown name indicates "dimethyl". Thus, dimethylsila-14-crown-5 may be abbreviated sila-14-crown-5.
The silacrowns are generally colorless, odorless liquids of moderate viscosity. They appear to have the ability to form stable molecular complexes with alkaline or alkaline earth salts in solution as well as in the solid state, behaving as phase transfer catalysts. Solvation of the metal ions leaves anions unencumbered, enabling them to act as potent bases and nucleophiles. This is demonstrated in a number of processes including nitrile, acetate, nitrite, fluoride and iodide displacements. Oxidations with permanganate and chromate are facilitated.
Certain silacrowns have the ability to react with siliceous materials, forming immobilized silacrowns. The immobilized silacrowns demonstrate the same ability to catalyze reactions as their unbound counterparts. They are particularly useful in liquid/liquid phase transfer reactions.